In order for hydrocarbon oils, particularly lube and transformer oils derived from petroleum oil distillates, to function effectively as lubricants or insulators under low temperature conditions, it is essential that the oils be free from wax. In the industry this dewaxing is conducted employing a variety of processes, the simplest being a reduction in temperature of the oil in question until the wax therein crystallize or solidifies at which point it can be removed from the oil by suitable separation procedures, such as filtration, centrifugation, etc. This procedure works well for light oils, but heavier oil distillates, bright stocks or residuum require solvent dilution in order to be dewaxed to a low enough pour point while retaining sufficient fluidity to facilitate handling. Typical solvents used in these solvent dewaxing processes include ketones, aromatic hydrocarbons, halogenated hydrocarbons and mixtures thereof. This solvent dewaxing can be practiced in a number of ways. It is well known that wax-containing petroleum oil stocks can be dewaxed by shock chilling with a cold solvent. It is also known that shock chilling, in itself, results in a low filtration rate of the dewaxed oil from the resultant wax/oil-solvent slurry. Because of this, the conventional method of solvent dewaxing wax-containing petroleum oil stocks has been cooling in scraped surface heat exchangers using an incremental solvent addition technique. In this technique, the dewaxing solvent is added at several points along the chilling apparatus. The waxy oil is chilled without solvent until some wax crystallization has occurred and the mixture is thickened considerably. The first increment of solvent is introduced at this point and cooling continues. Each incremental portion of solvent is added as necessary to maintain fluidity until the desired filtration temperature is reached at which point the remainder of the solvent required to obtain the proper viscosity of the mixture for filtration is added. In using this technique it is well known that the temperature of the incrementally added solvent should be the same as that of the main stream of oil at the point of addition to avoid the shock chilling effect.
Alternatively, the waxy oil can have cold solvent mixed with it and thereby be chilled to the wax separation temperature. A preferred embodiment of this direct dilution chilling procedure is described in U.S. Pat. No. 3,773,650. The procedure described therein, referred to as DILCHILL, avoids the adverse effects of shock chilling by introducing the waxy oil into a staged chilling zone and passing the waxy oil from stage to stage of the zone, while at the same time injecting cold dewaxing solvent into a plurality of the stages and wherein a high degree of agitation is maintained in the stages so as to effect substantially instanteous mixing of the waxy oil and solvent. As the waxy oil passes from stage to stage of the cooling zone, it is cooled to a temperature sufficiently low to precipitate wax therefrom without incurring the shock chilling effect. This produces a wax/oil-solvent slurry wherein the wax particles have a unique crystal structure which provides superior filtering characteristics such as high filtration rates of the dewaxed oil from the wax and high dewaxed oil yields.